The differentiation of myoblasts into myotubes is a complex process and can be inhibited by a variety of ways. These include activation of the Ras pathway and ligation of the Notch receptor. However, despite over a decade of research, the molecular mechanisms underlying such inhibition remain obscure. We have evidence that Notch signaling in the myogenic cell line C2C12 results in the hyperphosphorylation of the MAPK ERK. Interestingly, this appears to be due to a decrease in the levels of the MAPK phosphatase MKP-1. The goal of this research proposal is to determine how activated Notch inhibits myogenesis. We will initially focus our efforts on determining if activated Notch inhibits the initiation of myogenesis through phospho-ERK. Additionally, we will analyze the myogenin promoter to more fully define the mechanisms by which Notch inhibits muscle differentiation.